M0, M1, M2, M30: Program Stops and Program End

Four codes control when a program pauses and how it ends:

• M0 — stop, unconditionally. Machine holds; cycle-start resumes.
• M1 — optional stop: identical to M0, but only honored when the operator's optional stop switch is on. Otherwise skipped.
• M2 — program end.
• M30 — program end + rewind to the first line, armed for the next run. What modern posts emit.

...
M9               ; coolant off
M5               ; spindle stop
G53 G0 Z0        ; retract
G53 G0 X-10 Y200 ; present the part toward the operator
M30              ; end, rewind, ready for the next part

#Stops: M0 and M1

M0 is a scheduled interruption: flip the part over, blow out a pocket, insert a bushing. The program is the checklist — the stop happens at exactly the right line every time.

M1 is the prove-out tool. Posts put one before each tool change; with the optional-stop switch on, a new job runs one tool at a time with an inspection hold between. Once the job is trusted, switch off and M1 lines vanish.

The critical caveat for both: what the machine does to spindle and coolant during a stop is control-dependent. Some stop everything, some keep the spindle turning behind the closed door. If hands go anywhere near the work during the stop, the program must command M5/M9 first — never trust the stop code to do it.

#Ends: M2 and M30

The practical difference on modern controls is small: M30 rewinds to the top and generally resets more modal state (offsets stay; things like G91 or an active canned cycle don't survive on most controls — the exact reset list varies). Because "ready for the next part" is what production wants, M30 is the standard ending and M2 mostly appears in legacy files. Mach adds M47 — jump back to the first line without stopping — for continuous demo/warmup loops; treat it as the special case it is.

One habit worth stealing from good posts: end programs with the machine in a boring state — coolant off, spindle stopped, Z high, table presented forward — so the ending is identical whether the file ran clean or an operator hit reset halfway.

#Common mistakes

• Trusting M0 to make the machine safe. It stops the program, not necessarily the spindle. Explicit M5 M9 before any stop a human interacts with.
• No end code at all. A file that just runs out of lines behaves differently per control — some stop cleanly, some alarm. End with M30.
• Restarting after M2 on an old control and inheriting modal leftovers — a G91 or active cycle from the previous run. M30's fuller reset (and a proper preamble at the top) covers both directions.
• Using M0 for tool changes on a changer-equipped machine. That's what M6 is for; M0 leaves the change untracked and the offsets unmanaged.